What is continuous glucose monitoring?
Continuous glucose monitoring (CGM) gives information about glucose levels every few minutes, allowing the user to see a graph of glucose levels rather than just a single measurement at a given point in time.
The user or their carer inserts a sensor that measures the glucose level in the tissue just below the skin. The CGM sensor connects to a transmitter that beams the glucose data to a receiver, which can be either a handheld device or an insulin pump. The sensor is worn for six or more days and then replaced with a new one.
Watch our one and a half minute video to find out about continuous glucose monitors and how they can help you or your child manage blood glucose levels for type 1 diabetes.
What are the benefits of continuous glucose monitoring?
Continuous glucose monitoring allows you to track whether your glucose is high or low, stable, rising or falling. You can also see how your glucose levels vary. For example, you can track what happens while you are sleeping, after you eat, when you exercise, or when you are feeling unwell. The information is particularly helpful if you take insulin as you can match your insulin more easily to your needs.
There is a growing body of research that suggests continuous glucose monitoring can help reduce A1C without increasing the risk of hypoglycemia. (See our research section for more information). In October 2011 the Endocrine Society issued a new clinical guideline. They concluded that there is high quality evidence that continuous glucose monitoring can be a beneficial tool to help maintain target blood glucose levels, and limit the risk of hypoglycaemia in children and adolescents with type 1 diabetes who are at least eight years old, and in adults with type 1 diabetes as well if used on a daily basis.
Continuous glucose monitoring is potentially useful to anyone with diabetes, but is especially useful if you use insulin. CGM can be very helpful to people on multiple daily injections as well as to pump users. Some CGM users report their CGM is as beneficial to their glucose control as having a pump. It helps if you are motivated as CGM takes a while to get used to, and is not always straightforward to use. It’s also worth remembering that you’ll still have to do finger stick blood tests as well. Indeed some people using CGM say they do more finger stick tests than they used to!
How does continuous glucose monitoring work?
CGM works by sensing glucose levels in the body’s interstitial fluid. (Interstitial fluid is the fluid between tissue cells). These are tested every few minutes or so and the results sent to a receiver. In most systems, the results can be seen immediately.
A continuous glucose monitor has three parts:
- A sensor: this is inserted into the skin and worn for a few days, then disposed of. It senses how much glucose there is in the interstitial fluid. A sensor is usually around 5mm long.
- A transmitter: the sensor connects to a transmitter worn on the skin which communicates with the receiver (usually wirelessly).
- A receiver: the receiver records results and in most cases displays them immediately on a receiver. The latest systems can use a compatible mobile phone as the receiver, saving cost and the need to carry an extra device. Systems that display immediate results are often called ‘real-time’.
If you’re using a real-time system you can use the information to act swiftly to help avoid high or low blood sugar. If you download results for viewing on a computer later it can help you identify trends. It is particularly helpful for looking at night-time patterns, when it’s difficult to do blood stick tests.
Important information about CGMs
CGM sensors measure glucose levels in interstitial fluid and not blood glucose. Glucose levels in interstitial fluid lag behind glucose levels in blood. As a result, CGM results may be inaccurate, particularly when glucose levels are changing rapidly. For this reason, it’s sometimes necessary to take a blood stick test to confirm the reading before taking action for example dealing with a hypo alert when you don’t feel hypo. Some CGM systems also have to be calibrated with blood glucose levels (typically twice a day). CGM systems are most useful for viewing the glucose trend, i.e. is your glucose level stable, rising or falling.
For drivers, the DVLA says that group 1 drivers (cars and motorcycles) using continuous glucose monitoring can now rely on the CGM result for driving except in certain circumstances, such as if the reading is below 4mmol/L, when they will need to perform a finger stick blood glucose measurement.
What are the costs?
The costs associated with CGM include an initial outlay for a receiver (unless you have a compatible mobile phone), and the ongoing running costs (sensors, transmitters, adhesive covers if necessary, batteries for the receiver). The cost varies according to which system you buy. Please follow the links below to the CGM websites listed below for up-to-date costs and any special offers. Most sensors cost in the region of £40 – £60 each. You might want to consider the Abbott Freestyle Libre, which is not CGM but will allow you to access continuous glucose data.
CGM systems available in the UK
- You are likely to get the most benefit from CGM if you have support from your diabetes team
- Standalone systems can be used with any pump or with MDI
- Standalone systems show the results on a receiver or compatible mobile phone
- Integrated systems show the results on the screen of the integrated pump
- It is possible to pay the initial costs of a long-term CGM system yourself and receive full or partial NHS support for the sensors
Who supplies CGM systems in the UK?
There are various suppliers offering CGMs licensed for different age groups:
- Dexcom – G5 and G6 as a separate system or linked to the t:slim x2 insulin pump, licensed for people aged 2+
- GlucoMen Day – standalone system with a 14-day sensor, licensed for people aged 6+
- Medtronic – as a separate system or linked to the Veo, 640G, 670G or 780G insulin pumps systems. The 640G and Guardian Connect standalone system are licensed for all ages whilst 670G is licensed for those aged 7+ and using 8+ units of insulin per day. Medtronic also makes a system (iPro 2) which records CGM readings for later reading on a computer, without displaying it to the wearer. This is for short term diagnostic use.
- Medtrum A6 – standalone or integrated. Licensed for people aged 2+
Please note that:
- Real time CGM has alarms to indicate high or low glucose or rapid change. Some systems provide early warning alarms if glucose levels are likely to fall too high or low. Alarms can usually be set by the user at the level they want. There may be a choice of an audible or vibrating alarm.
- Flash glucose monitoring – Libre does not transmit and cannot alarm for a high or low result until you scan the sensor. However, Libre 2, coming in 2021, will alert you to the need to perform a scan when your glucose goes out of target range.